Generation of ROS in MII oocytes during female and postovulatory aging. (A) Fluorescent microscope images of hydroethidium (HEt)-labeled oocytes. (B) Percentage of HEt-positive oocytes (N: total number of oocytes analyzed). Reproductively-aged females exhibited a significantly higher percentage of HEt-positive oocytes, indicating increased ROS levels, than oocytes from young females. Furthermore, in vivo and in vitro postovulatory-aged oocytes showed significantly increased ROS levels compared to fresh oocytes from young females. ****P < 0.0001.